Bulletin - United States Geological Survey, Τεύχη 150-152

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Σελίδα 279 - The most striking feature; in the texture of the unaltered gabbro is the repeated and abrupt change in the coarseness of the grain which is seen at ноте localities.
Σελίδα 49 - ... or layers. On account of their sudden cooling, many volcanic rocks are either wholly glassy or only partially crystallized. Undoubtedly the rhyolite in this vicinity was originally only a partially crystallized rock when brought to the surface. A rough examination of the hand specimens is sufficient to show that the rock consists of two parts : one part the fine groundmass, or background, and the other the...
Σελίδα 116 - ... perpendicular, and as white and almost as pure as the celebrated chalk cliffs of Dover, England. Their remoteness from the lines of travel is the probable explanation of their having so long been overlooked by American geologists. The chalk of these cliffs scales off rapidly in great conchoidal flakes, and, owing to the irregularity of this process, its face instead of being a continuous plane, is composed of many acute and reentrant angles, resembling the bastions of a fortress. The summit of...
Σελίδα 377 - ... distributed the abundant folia of black mica. In the thin section the structure is cataclastic; that is to say, the various minerals of which it is composed have, since their formation, been more or less shattered and distorted through dynamic agencies acting upon the rock mass of which they form a part. Quartz and black mica are the most prominent constituents, though there are abundant feldspars of both potash and soda lime varieties. In addition to these are abundant accessory epidotes, a...
Σελίδα 289 - Tb'rnebohm (see above, p. 43) are finely developed. The interior zone of these rims next to the olivine is narrow, granular, and almost colorless. Outside of this is a darker greenish portion, having a feathery appearance; this exterior zone projects in rounded tufts into the feldspar substance, at the expense of which it has apparently been formed.
Σελίδα 110 - The calcite crystals are worthy of special cristallographie study. The occurrence of pyrites shows that in some cases its formation was simultaneous with that of calcite, while in other instances it was apparently subsequent to it. The elongated crystals of tarnished pyrites are quite remarkable and might easily be confounded with rutile; but they show a yellow color and a cross fracture, and u blowpipe examination reveals their real character.
Σελίδα 108 - The outcrop along the Mazon appears nearly continuous, but still I have not been able to satisfy myself as to the connection of the above beds with those of the lower part of the stream. The strata there developed consist of very variable sandy clay shales and sandstones, in some places becoming nearly pure clay shales, but containing many nodules of carbonate of iron. Pine Bluff, at the lowermost crossing of the Mazon, is composed of about forty feet of heavily bedded, but rather fissile sandstone,...
Σελίδα 151 - The high limit of the extinction angles, as well as the chemical composition of the rock and the spherulites, since there is no evidence of the presence of more than one species of feldspar, indicates that the orthoclase is rich in soda, the molecular ratio of the potash to soda in the rock being 1 to 1. There is a difference between the end of the projecting prisms of feldspar and the part of the ray within the spherulite.
Σελίδα 318 - ... component is a silky white mica (sericite or kaolin), whose individual scales vary greatly in size in different specimens. This is sometimes wholly or in part replaced by green chlorite, which gives rise to subordinate intercalated beds of chlorite slate or schist. Quartz grains of varying size and outline are generally present, while feldspar is extremely rare. It is probably the alteration of this mineral in the process of weathering which has given rise to the abundant white mica. Iron oxide...
Σελίδα 377 - ... to the weathering, the first mentioned being judged as largely post-Cretaceous and the last post-Glacial. Materials of the nature here described are subject to considerable variation, and the descriptions given must be accepted as descriptive of the general results produced rather than as definitely applicable to any particular sample. The fresh rock, illustrated by the small fragment in the box with the residual sand. No. 147, is a strongly foliated micaceous granite, the foliation being secondary...

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